Objective: The main goal of this application is to study the mechanism whereby the low density lipoprotein receptor-related protein 1 (LRP1) modulates the permeability of the BBB under physiological conditions and after an ischemic stroke. Methods In Aim I we will test the hypothesis that under non-ischemic conditions astrocytic LRP1 preserves the integrity of the basement membrane-astrocyte (BM-A) interface by promoting the recruitment of integrins to the membrane of perivascular astrocytic end- feet processes and their binding to its ligands in the basement membrane (BM). In Aim II we propose that this function is lost under ischemic conditions when the cleavage of the extracellular domain of LRP1 from perivascular astrocytes activates a cell-signaling pathway that increases the permeability of the blood-brain barrier (BBB). In Aim III we will study in an animal model of ischemic stroke whether inhibition of LRP1 cleavage from perivascular astrocytes with the receptor-associated protein (RAP) prevents the harmful effects of endogenous tPA and/or rtPA on the permeability of the BBB. Research Plan In the first Aim of this application we will use astrocytic cell cultures and a cell adhesion assay. In the second Aim we will use astrocytic cultures, an in vitro model of the blood- brain barrier, real-time PCR, confocal microscopy and co-immunoprecipitation assays. In the third Aim we will use a nanocarrier that we engineered to selectively deliver its content to the BBB in the ischemic tissue and an animal model of cerebral ischemia.